Connective tissue is the framework upon which other tissue types, i.e., epithelial, muscle, and nervous tissues, are supported. Connective tissue generally includes individual cells not directly attached to one another and held within the extracellular matrix. The extracellular matrix, in turn, includes the ground substance, e.g., the minerals of bone, the plasma of blood, etc.; cell and tissue supporting materials including glycosaminoglycans (GAGs); and structural proteinaceous components including collagen fibers and elastin fibers. Glycosaminoglycans (GAGs) are long unbranched polysaccharides present in the connective tissue layer of many materials suitable for forming implants. GAGs promote cell proliferation, wound repair and tissue hydration and can act as a shock absorber or cushioning agent in a tissue.
Connective tissue can assume widely divergent architectures, ranging from blood, in which the fibrous component is absent and the ground substance is fluid, to dense connective tissue, which includes a relatively high proportion of extracellular fibers (e.g., collagen) and may contain low concentrations of other connective tissue components. There are many specialized types of connective tissue, one example being elastic tissue, in which elastic fibers are the major component of the tissue and the amount of factors commonly found in other types of connective tissue, such as collagen and proteoglycans, may be minimal.
The degeneration of the structural proteins in connective tissue is associated with many pathologic conditions include aneurysm, Marfan syndrome, supravalvular aortic stenosis, and chronic obstructive pulmonary disease (COPD). For those afflicted, such conditions lead to, at the very least, a lowered quality of life. Current methods of treatment for degeneration of connective tissue are limited. For instance, aneurysm treatment methods are often limited to invasive surgical techniques including stent repair or vascular graft. Unfortunately, surgical solutions include high risk of complication due to, e.g., neurological injuries, bleeding, or stroke as well as implant-related complications such as thrombosis, leakage or implant failure.
Accordingly, methods and materials that can provide stabilization of connective tissue would be highly beneficial.
Connective tissue is also commonly present in implantable bioprosthetics. For instance, bioprosthetic heart valves, which consist primarily of bovine pericardium and porcine aortic heart valve, are utilized in forming bioprosthetic heart valves for replacement of damaged natural valves. Bioprosthetic heart valves include allograft valves, which include biomaterial supplied from human cadavers; autologous valves, which include biomaterial supplied from the individual receiving the valve; and xenograft valves, which include biomaterial obtained from non-human biological sources including pigs, cows or other animals. Bioprosthetic heart valves can be used to replace damaged or diseased heart valves including aortic, mitral, and pulmonary valves.
Allograft transplants have been quite effective, with good compatibility and blood flow characteristics in the recipients. However, the availability of human valves for transplantation continues to decline as a percentage of cardiac surgeries performed each year. As such, the choice of xenograft materials for use in replacement BPHVs is becoming more common.
Both xenografts and allografts require that the graft biomaterial be stabilized via chemical fixation prior to use in order to render the biomaterial more non-antigenic as well as to improve resistance of the biomaterial to degradation. Currently, glutaraldehyde fixation of implantable biomaterial is used. Glutaraldehyde fixation forms covalent cross-links between free amines in the collagen of the connective tissue. Glutaraldehyde is commonly used alone as well as in combination with a variety of other compounds in stabilizing tissues for implant. For instance, traditional glutaraldehyde fixation methods are adequate for fixing collagen, but this method is not adequate for fixing other extra cellular matrix components of a tissue. For example, GAGs are not fixed via glutaraldehyde crosslinking regimes. As GAGs of the spongiosa layer can act as a cushion between the outer fibrosa and ventricularis layers during function, the leaching of GAGs from implantable materials can lead to reduced bending stiffness and ultimately to degenerative failure of the implant. Attempts have been made to stabilize GAGs in implantable tissues. While these methods have shown some success in preventing degradation of implant materials, room for improvement exists.
Stabilization regimes used alone or in conjunction with glutaraldehyde fixation protocols include use of polyepoxy amines for crosslinking a variety of amino acid residues found in tissue proteins (see, e.g., U.S. Pat. No. 6,391,538 to Vyavahare, et al., which is incorporated herein by reference), use of phenolic tannins for elastin fixation (see, e.g., U.S. Pat. No. 7,713,543 to Vyavahare, et al., which Is Incorporated herein by reference), and use of various chemistries including carbodiimide chemistry for stabilization of glycosaminoglycans in biological tissues (see, e.g., U.S. Pat. No. 6,861,211 to Levy, et al., U.S. Pat. No. 7,918,899 to Girardot, et al., and U.S. Pat. No. 7,479,164 to Girardot, et al., which are incorporated herein by reference).
Despite advances in addressing the needs for longer lasting and better performing implantable bioprosthetics, there remains room for variation and improvement within the art.